1. Field of the Invention
The present invention relates in general to a digital data recovery circuit and, specifically, to a data recovery circuit for deriving digital signals from video signals and digital control signals that are recorded by frequency multiplexing.
2. Description of the Prior Art
Cameras have been proposed that can produce still images, not by the use of conventional silver-halide photography, but by using video tape recorder technology. Such still cameras of the video recorder kind must be portable and light weight and one approach to achieving such desired capabilities is by using a solid-state image pick-up sensor and a magnetic disc to record the image information. The still video signals or image signals, as well as other relevant digital data such as the date, title, name, and the like, are recorded using frequency multiplex techniques as concentric video recording tracks on the magnetic disc. In one proposed still-image video recording scheme, the digital data is recorded in a frequency band that is different than that chosen for the video image signals, and such digital data is recorded by phase-shift-keying (PSK) modulation, which permits fewer read errors during reproduction of such digital data. In this approach, a "1" bit of the digital data signal is represented by the positive phase of a carrier signal and a "0" bit is represented by the negative phase of that carrier waveform.
In the circuit proposed for use in deriving the digital data recorded on the video magnetic disc using the above-described multiplexing technique, a continuous demodulation carrier is generated that is based on the carrier signals originally modulated by the digital data. The generated continuous carrier and the reproduced PSK-modulated signal are multiplied together following the well-known synchronous detection technique, thereby demodulating the digital data, which consists of "1" bits and "0" bits. The oscillator that is used to generate the continuous carrier signal for such demodulation is controlled by a phase-locked-loop (PLL), so that the output signal from the oscillator is phased locked onto the reproduced PSK signal.
In this approach, it is a requirement that the PSK-modulated digital data signal must be recorded at a very low signal level to prevent the video image signals and the PSK-modulated data signals from interfering with each other. The video image signals are, of course, of utmost importance. Therefore, the reproduction circuit must employ a loop filter that has a relatively long time constant, so as to reduce the adverse effects of noise on this low level signal. Because of this long time constant, the phase loop of the reproducing circuit has a poor response characteristic relative to phase variations and when there is an abrupt shift in phase of the carrier of the PSK signal, such as might occur at a switching point of a single track due to irregular rotation of the disc during recording or during playback, the output phase of the continuous carrier oscillator cannot respond to such a phase shift, and this results in errors in the data being read out.